Effects of Wall Heat Loss on Swirl-Stabilized Nonpremixed Flames With Localized Extinction

2018 ◽  
Vol 140 (12) ◽  
Author(s):  
Huangwei Zhang
Keyword(s):  
Heat Flux ◽  
Heat Loss ◽  
Bluff Body ◽  
Flame Stabilization ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Special Focus ◽  
Turbulent Heat ◽  
Total Enthalpy ◽  
The Mean

Large eddy simulation (LES) with three-dimensional conditional moment closure (CMC) subgrid model for combustion is applied to simulate a swirl-stabilized nonpremixed methane flame with localized extinction, with special focus on the effects of heat loss to the burner surface. The convective wall heat loss is modeled through introducing a source term in the conditionally filtered total enthalpy equation for the CMC cells adjacent to the wall. The mean heat flux is high on the middle surface of the bluff body, but relatively low near its edges. The turbulent heat flux based on the gradient of the resolved temperature is relatively low compared to the laminar counterpart, but increases with the turbulent intensity. The heat loss facilitates the occurrences of extinction and re-ignition for the CMC cells immediately adjacent to the wall, evidenced by comparing flame structures in the near-wall CMC cells. This can be directly linked to the increase of the mean conditional scalar dissipation near the wall in the heat loss case. Furthermore, the degree of local extinction near the bluff body measured by conditional reactedness at stoichiometry is intensified due to the wall heat loss. However, the results also show that there is negligible influence of wall heat loss on the probability density function (PDF) of the lift-off height, demonstrating the dominance of aerodynamic effects on flame stabilization. The results are in reasonable agreement with experimental measurements.

scholarly journals Modelling of a Bluff-Body Stabilised Premixed Flames Close to Blow-Off

Computation ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 43
Author(s):  
Shokri Amzin ◽  
Mohd Fairus Mohd Yasin
Keyword(s):  
Bluff Body ◽  
Fluid Dynamic ◽  
Premixed Flames ◽  
Pollutant Emissions ◽  
Moment Closure ◽  
Recirculation Region ◽  
Scalar Dissipation ◽  
Average Error ◽  
Conditional Moment ◽  
Lean Premixed

As emission legislation becomes more stringent, the modelling of turbulent lean premixed combustion is becoming an essential tool for designing efficient and environmentally friendly combustion systems. However, to predict emissions, reliable predictive models are required. Among the promising methods capable of predicting pollutant emissions with a long chemical time scale, such as nitrogen oxides (NOx), is conditional moment closure (CMC). However, the practical application of this method to turbulent premixed flames depends on the precision of the conditional scalar dissipation rate,. In this study, an alternative closure for this term is implemented in the RANS-CMC method. The method is validated against the velocity, temperature, and gas composition measurements of lean premixed flames close to blow-off, within the limit of computational fluid dynamic (CFD) capability. Acceptable agreement is achieved between the predicted and measured values near the burner, with an average error of 15%. The model reproduces the flame characteristics; some discrepancies are found within the recirculation region due to significant turbulence intensity.

An explicit algebraic Reynolds-stress and scalar-flux model for stably stratified flows

2013 ◽  
Vol 723 ◽  
pp. 91-125 ◽  
Author(s):  
W. M. J. Lazeroms ◽  
G. Brethouwer ◽  
S. Wallin ◽  
A. V. Johansson
Keyword(s):  
Heat Flux ◽  
Kinetic Energy ◽  
Temperature Gradient ◽  
Reynolds Stress ◽  
Turbulent Heat Flux ◽  
Turbulent Heat ◽  
Homogeneous Shear ◽  
Self Consistent ◽  
The Mean ◽  
Homogeneous Shear Flow

AbstractThis work describes the derivation of an algebraic model for the Reynolds stresses and turbulent heat flux in stably stratified turbulent flows, which are mutually coupled for this type of flow. For general two-dimensional mean flows, we present a correct way of expressing the Reynolds-stress anisotropy and the (normalized) turbulent heat flux as tensorial combinations of the mean strain rate, the mean rotation rate, the mean temperature gradient and gravity. A system of linear equations is derived for the coefficients in these expansions, which can easily be solved with computer algebra software for a specific choice of the model constants. The general model is simplified in the case of parallel mean shear flows where the temperature gradient is aligned with gravity. For this case, fully explicit and coupled expressions for the Reynolds-stress tensor and heat-flux vector are given. A self-consistent derivation of this model would, however, require finding a root of a polynomial equation of sixth-order, for which no simple analytical expression exists. Therefore, the nonlinear part of the algebraic equations is modelled through an approximation that is close to the consistent formulation. By using the framework of a$K\text{{\ndash}} \omega $model (where$K$is turbulent kinetic energy and$\omega $an inverse time scale) and, where needed, near-wall corrections, the model is applied to homogeneous shear flow and turbulent channel flow, both with stable stratification. For the case of homogeneous shear flow, the model predicts a critical Richardson number of 0.25 above which the turbulent kinetic energy decays to zero. The channel-flow results agree well with DNS data. Furthermore, the model is shown to be robust and approximately self-consistent. It also fulfils the requirements of realizability.

scholarly journals Dynamics of lean premixed flames stabilized on a meso-scale bluff-body in an unconfined flow field

2018 ◽  
Vol 13 (6) ◽  
pp. 48 ◽  
Author(s):  
Yu Jeong Kim ◽  
Bok Jik Lee ◽  
Hong G. Im
Keyword(s):  
Bluff Body ◽  
Proper Time ◽  
Premixed Flames ◽  
Narrow Channel ◽  
Flame Stabilization ◽  
Inflow Velocity ◽  
Lean Premixed Flames ◽  
Lean Premixed ◽  
The Mean ◽  
Meso Scale

Two-dimensional direct numerical simulations were conducted to investigate the dynamics of lean premixed flames stabilized on a meso-scale bluff-body in hydrogen-air and syngas-air mixtures. To eliminate the flow confinement effect due to the narrow channel, a larger domain size at twenty times the bluff-body dimension was used in the new simulations. Flame/flow dynamics were examined as the mean inflow velocity is incrementally raised until blow-off occurs. As the mean inflow velocity is increased, several distinct modes in the flame shape and fluctuation patterns were observed. In contrast to our previous study with a narrow channel, the onset of local extinction was observed during the asymmetric vortex shedding mode. Consequently, the flame stabilization and blow-off behavior was found to be dictated by the combined effects of the hot product gas pocket entrained into the extinction zone and the ability to auto-ignite the mixture within the given residence time corresponding to the lateral flame fluctuations. A proper time scale analysis is attempted to characterize the flame blow-off mechanism, which turns out to be consistent with the classic theory of Zukoski and Marble.

Convective Heat Transfer in Wall-Bounded Flows Affected by Severe Fluid Properties Variation: A Second-Moment Closure Study

2010 ◽  
Author(s):  
S. Jakirlic´ ◽  
R. Jester-Zu¨rker
Keyword(s):  
Heat Flux ◽  
Dissipation Rate ◽  
Transport Equations ◽  
Moment Closure ◽  
Reference Database ◽  
Turbulent Heat ◽  
Second Moment ◽  
Fluid Properties ◽  
Second Moment Closure ◽  
Near Wall

Different flow configurations subjected to increasingly enhanced wall heating were selected to be computationally investigated by means of a differential, near-wall second-moment closure model based on the solution of transport equations for second moments of the fluctuating velocities and temperature, ui″uj″͠ and ui″θ͠ respectively. Both Reynolds stress model and heat flux model represent wall-topography free formulations with quadratic pressure-strain term and pressure-temperature-gradient correlation. The transport equations for the turbulent stress tensor and the turbulent heat flux are solved in conjunction with the equation governing a new scale supplying variable, so-called “homogeneous” dissipation rate, Jakirlic and Hanjalic (2002). Such an approach offers a number of important advantages: proper near-wall shape of the dissipation rate profile was obtained without introducing any additional term and the correct asymptotic behaviour of the stress dissipation components by approaching the solid wall is fulfilled automatically without necessity for any wall geometry-related parameter. The configurations considered include fully-developed and developing flows in channel (without and with a sudden expansion) and pipe in conjunction with the scalar transport under conditions of variable fluid properties for which an extensive experimental and numerical (DNS and LES) reference database exists.

A Comparison of Southern Ocean Air–Sea Buoyancy Flux from an Ocean State Estimate with Five Other Products

2011 ◽  
Vol 24 (24) ◽  
pp. 6283-6306 ◽  
Author(s):  
Ivana Cerovečki ◽  
Lynne D. Talley ◽  
Matthew R. Mazloff
Keyword(s):  
Heat Flux ◽  
Southern Ocean ◽  
Heat Loss ◽  
Large Scale ◽  
Heat Fluxes ◽  
Buoyancy Flux ◽  
Western Boundary ◽  
Turbulent Heat ◽  
Radiative Fluxes ◽  
State Estimate

Abstract The authors have intercompared the following six surface buoyancy flux estimates, averaged over the years 2005–07: two reanalyses [the recent ECMWF reanalysis (ERA-Interim; hereafter ERA), and the National Centers for Environmental Prediction (NCEP)–NCAR reanalysis 1 (hereafter NCEP1)], two recent flux products developed as an improvement of NCEP1 [the flux product by Large and Yeager and the Southern Ocean State Estimate (SOSE)], and two ad hoc air–sea flux estimates that are obtained by combining the NCEP1 or ERA net radiative fluxes with turbulent flux estimates using the Coupled Ocean–Atmosphere Response Experiment (COARE) 3.0 bulk formulas with NCEP1 or ERA input variables. The accuracy of SOSE adjustments of NCEP1 atmospheric fields (which SOSE uses as an initial guess and a constraint) was assessed by verification that SOSE reduces the biases in the NCEP1 fluxes as diagnosed by the Working Group on Air–Sea Fluxes (Taylor), suggesting that oceanic observations may be a valuable constraint to improve atmospheric variables. Compared with NCEP1, both SOSE and Large and Yeager increase the net ocean heat loss in high latitudes, decrease ocean heat loss in the subtropical Indian Ocean, decrease net evaporation in the subtropics, and decrease net precipitation in polar latitudes. The large-scale pattern of SOSE and Large and Yeager turbulent heat flux adjustment is similar, but the magnitude of SOSE adjustments is significantly larger. Their radiative heat flux adjustments patterns differ. Turbulent heat fluxes determined by combining COARE bulk formulas with NCEP1 or ERA should not be combined with unmodified NCEP1 or ERA radiative fluxes as the net ocean heat gain poleward of 25°S becomes unrealistically large. The other surface flux products (i.e., NCEP1, ERA, Large and Yeager, and SOSE) balance more closely. Overall, the statistical estimates of the differences between the various air–sea heat flux products tend to be largest in regions with strong ocean mesoscale activity such as the Antarctic Circumpolar Current and the western boundary currents.

Pseudo-turbulent heat flux and average gas–phase conduction during gas–solid heat transfer: flow past random fixed particle assemblies

2016 ◽  
Vol 798 ◽  
pp. 299-349 ◽  
Author(s):  
Bo Sun ◽  
Sudheer Tenneti ◽  
Shankar Subramaniam ◽  
Donald L. Koch
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Gas Phase ◽  
Volume Fraction ◽  
Turbulent Heat Flux ◽  
Turbulent Heat ◽  
Fluid Temperature ◽  
Bulk Fluid ◽  
The Mean ◽  
Temperature Equation

Fluctuations in the gas-phase velocity can contribute significantly to the total gas-phase kinetic energy even in laminar gas–solid flows as shown by Mehrabadi et al. (J. Fluid Mech., vol. 770, 2015, pp. 210–246), and these pseudo-turbulent fluctuations can also enhance heat transfer in gas–solid flow. In this work, the pseudo-turbulent heat flux arising from temperature–velocity covariance, and average fluid-phase conduction during convective heat transfer in a gas–solid flow are quantified and modelled over a wide range of mean slip Reynolds number and solid volume fraction using particle-resolved direct numerical simulations (PR-DNS) of steady flow through a random assembly of fixed isothermal monodisperse spherical particles. A thermal self-similarity condition on the local excess temperature developed by Tenneti et al. (Intl J. Heat Mass Transfer, vol. 58, 2013, pp. 471–479) is used to guarantee thermally fully developed flow. The average gas–solid heat transfer rate for this flow has been reported elsewhere by Sun et al. (Intl J. Heat Mass Transfer, vol. 86, 2015, pp. 898–913). Although the mean velocity field is homogeneous, the mean temperature field in this thermally fully developed flow is inhomogeneous in the streamwise coordinate. An exponential decay model for the average bulk fluid temperature is proposed. The pseudo-turbulent heat flux that is usually neglected in two-fluid models of the average fluid temperature equation is computed using PR-DNS data. It is found that the transport term in the average fluid temperature equation corresponding to the pseudo-turbulent heat flux is significant when compared to the average gas–solid heat transfer over a significant range of solid volume fraction and mean slip Reynolds number that was simulated. For this flow set-up a gradient-diffusion model for the pseudo-turbulent heat flux is found to perform well. The Péclet number dependence of the effective thermal diffusivity implied by this model is explained using a scaling analysis. Axial conduction in the fluid phase, which is often neglected in existing one-dimensional models, is also quantified. As expected, it is found to be important only for low Péclet number flows. Using the exponential decay model for the average bulk fluid temperature, a model for average axial conduction is developed that verifies standard assumptions in the literature. These models can be used in two-fluid simulations of heat transfer in fixed beds. A budget analysis of the mean fluid temperature equation provides insight into the variation of the relative magnitude of the various terms over the parameter space.

Characterization of Turbulent Heat Transfer in Ribbed Pipe Flow

2016 ◽  
Vol 138 (4) ◽  
Author(s):  
Changwoo Kang ◽  
Kyung-Soo Yang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pipe Flow ◽  
Temperature Fields ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Mean Flow ◽  
Transfer Enhancement ◽  
Pitch Ratio ◽  
The Mean

In the current investigation, we performed large eddy simulation (LES) of turbulent heat transfer in circular ribbed-pipe flow in order to study the effects of periodically mounted square ribs on heat transfer characteristics. The ribs were implemented on a cylindrical coordinate system by using an immersed boundary method, and dynamic subgrid-scale models were used to model Reynolds stresses and turbulent heat flux terms. A constant and uniform wall heat flux was imposed on all the solid boundaries. The Reynolds number (Re) based on the bulk velocity and pipe diameter is 24,000, and Prandtl number is fixed at Pr = 0.71. The blockage ratio (BR) based on the pipe diameter and rib height is fixed with 0.0625, while the pitch ratio based on the rib interval and rib height is varied with 2, 4, 6, 8, 10, and 18. Since the pitch ratio is the key parameter that can change flow topology, we focus on its effects on the characteristics of turbulent heat transfer. Mean flow and temperature fields are presented in the form of streamlines and contours. How the surface roughness, manifested by the wall-mounted ribs, affects the mean streamwise-velocity profile was investigated by comparing the roughness function. Local heat transfer distributions between two neighboring ribs were obtained for the pitch ratios under consideration. The flow structures related to heat transfer enhancement were identified. Friction factors and mean heat transfer enhancement factors were calculated from the mean flow and temperature fields, respectively. Furthermore, the friction and heat-transfer correlations currently available in the literature for turbulent pipe flow with surface roughness were revisited and evaluated with the LES data. A simple Nusselt number correlation is also proposed for turbulent heat transfer in ribbed pipe flow.

scholarly journals Processes Controlling Sea Surface Temperature Variability of Ningaloo Niño

2020 ◽  
Vol 33 (10) ◽  
pp. 4369-4389 ◽  
Author(s):  
Yaru Guo ◽  
Yuanlong Li ◽  
Fan Wang ◽  
Yuntao Wei ◽  
Zengrui Rong
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Heat Loss ◽  
Latent Heat ◽  
General Circulation ◽  
Sea Surface ◽  
Turbulent Heat ◽  
Sst Warming ◽  
Latent Heat Loss

AbstractA high-resolution (3–8 km) regional oceanic general circulation model is utilized to understand the sea surface temperature (SST) variability of Ningaloo Niño in the southeast Indian Ocean (SEIO). The model reproduces eight Ningaloo Niño events with good fidelity and reveals complicated spatial structures. Mesoscale noises are seen in the warming signature and confirmed by satellite microwave SST data. Model experiments are carried out to quantitatively evaluate the effects of key processes. The results reveal that the surface turbulent heat flux (primarily latent heat flux) is the most important process (contribution > 68%) in driving and damping the SST warming for most events, while the roles of the Indonesian Throughflow (~15%) and local wind forcing are secondary. A suitable air temperature warming is essential to reproducing the reduced surface latent heat loss during the growth of SST warming (~66%), whereas the effect of the increased air humidity is negligibly small (1%). The established SST warming in the mature phase causes increased latent heat loss that initiates the decay of warming. A 20-member ensemble simulation is performed for the 2010/11 super Ningaloo Niño, which confirms the strong influence of ocean internal processes in the redistribution of SST warming signatures. Oceanic eddies can dramatically modulate the magnitudes of local SST warming, particularly in offshore areas where the “signal-to-noise” ratio is low, raising a caution for evaluating the predictability of Ningaloo Niño and its environmental consequences.

IMPLEMENTATION OF THE CONDITIONAL MOMENT CLOSURE MODEL TO A TURBULENT NONPREMIXED H2/CO-AIR FLAME STABILIZED ON A BLUFF BODY

1999 ◽  
Vol 23 (3-4) ◽  
pp. 425-433 ◽  
Author(s):  
S.- H. Kim ◽  
T. Liu ◽  
K.Y. Huh
Keyword(s):  
Bluff Body ◽  
Reaction Rates ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Conditional Moment ◽  
Turbulent Diffusion Flame ◽  
Recirculating Flow ◽  
Conditional Moment Closure ◽  
Mass Fractions ◽  
Turbulent Nonpremixed

A turbulent nonpremixed flame of H2/CO-air stabilized on a bluff-body is simulated by the conditional moment closure (CMC) model. Full spatial variation of the conditional quantities is taken into account for an elliptic recirculating flow field. Comparison has shown reasonable agreement for the conditional and Favre mean temperature and mass fractions of CO and H20 between calculation and experiment. Overprediction of the peak OH mass fraction is attributed to inaccurate modelling of the conditional scalar dissipation rate. The CMC model is capable of predicting major features of a turbulent diffusion flame characterized by finite chemical reaction rates.

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